An arrangement in which a heat exchanger of an evaporator, which heats water to make vapor using exhaust gas heat, is disposed in the exhaust passage of an internal combustion engine is known from Japanese Utility Model Registration Publication No. 1-33768. In this arrangement, a plurality of heat transfer tubes wound in a coil shape are disposed within a zigzag passage through which the exhaust gas passes, and the water flowing through the interior of the heat transfer tubes is heated with high temperature exhaust gas so as to generate vapor.
The temperature of exhaust gas that has been discharged from an exhaust port of an internal combustion engine into an exhaust passage gradually decreases as it undergoes heat exchange with water flowing through the interior of a heat transfer tube of a heat exchanger. However, in the arrangement described in Japanese Utility Model Registration Publication No. 1-33768, since the plurality of heat transfer tubes are all of the same shape and are formed by winding pipe material having the same diameter with the same pitch, it is difficult to always obtain high heat exchange efficiency across regions where the temperature of the exhaust gas varies.